Fungi (Kingdom Fungi) are heterotrophic eukaryotes that obtain nutrition by absorption (secreting digestive enzymes externally and absorbing the breakdown products). Key characteristics: Cell wall composed of CHITIN (not cellulose as in plants, not peptidoglycan as in bacteria). Heterotrophic — no chlorophyll or photosynthesis. Body plan: usually filamentous, consisting of thread-like hyphae that collectively form mycelium (except unicellular yeasts). Reserve food: glycogen (like animals, not starch like plants). Reproduce sexually and asexually by spores. Cell membrane contains ergosterol (not cholesterol as in animals or phytosterols as in plants). Classified into: Phycomycetes (water moulds, downy mildews), Ascomycetes (sac fungi), Basidiomycetes (club fungi/mushrooms), Deuteromycetes (imperfect fungi, asexual reproduction only).

Fungal sexual reproduction involves three distinct processes that in most organisms are coupled but in fungi (particularly higher fungi) are temporally separated: Plasmogamy: The physical union of two compatible fungal cells, involving fusion of their plasma membranes and mixing of their cytoplasm (protoplasm). After plasmogamy, two haploid nuclei from the two parent cells coexist in the same cell. The resulting cell with two nuclei from different parents is called a dikaryon or heterokaryon; this condition (n+n) differs from a true diploid (2n) because the nuclei remain separate rather than fusing. Karyogamy: The fusion of the two haploid nuclei present in the dikaryon to form a single diploid nucleus (2n). Karyogamy is the event that actually creates the zygote nucleus. In lower fungi (Phycomycetes like Rhizopus and Mucor), karyogamy follows almost immediately after plasmogamy with no prolonged dikaryotic phase. In higher fungi (Ascomycetes and Basidiomycetes), karyogamy is delayed, and the dikaryotic phase may persist through extensive mycelium growth before karyogamy occurs in specific cells. Meiosis: The diploid nucleus produced by karyogamy undergoes meiotic division, restoring haploidy and producing genetically diverse haploid spores. Meiosis in fungi occurs in specialised structures — the ascus (in Ascomycetes) or the basidium (in Basidiomycetes).

The dikaryotic phase, a prolonged period between plasmogamy and karyogamy where cells contain two separate haploid nuclei, is a distinctive and fascinating feature of the life cycles of Ascomycetes and Basidiomycetes. In Basidiomycetes (mushrooms), the secondary mycelium (the main mycelium from which the fruiting body develops) is dikaryotic — each cell contains two nuclei, one from each mating type, that divide simultaneously in a process called conjugate division, with a special mechanism called "clamp connections" ensuring each daughter cell receives one nucleus of each mating type. The mushroom fruiting body (what we eat) is composed entirely of this dikaryotic mycelium. Karyogamy and meiosis occur only in the basidia (specialised cells in the gills of the mushroom cap), producing four haploid basidiospores on each basidium. In Ascomycetes (yeasts, morels, truffles, Neurospora), the dikaryotic phase is typically briefer, confined to the hyphae that form the ascoma (fruiting body), with karyogamy and meiosis occurring in asci. The biological significance of the extended dikaryotic phase is debated — it may allow genetic variation to be "tested" before committing to the irreversible fusion of karyogamy and the genetic recombination of meiosis.

The three events of plasmogamy, karyogamy, and meiosis are universally required for sexual reproduction in eukaryotes, but their relative timing varies significantly. In most animals: plasmogamy (sperm-egg membrane fusion) → karyogamy (pronuclear fusion at fertilisation) → meiosis (occurs in germ cells before fertilisation, before sperm and egg formation). In plants: meiosis occurs early (producing spores) → spores develop into gametophytes → gametophytes produce gametes → plasmogamy (gamete fusion) → karyogamy → zygote → sporophyte. In fungi: plasmogamy (hyphal fusion) → extended dikaryotic phase (unique to higher fungi) → karyogamy → meiosis → haploid spores → new haploid mycelium. This comparison illustrates that while the same fundamental events occur in all sexually reproducing eukaryotes, evolution has arranged these events in different sequences and temporal relationships in different organism groups, with the fungal arrangement (particularly the extended dikaryotic phase between plasmogamy and karyogamy) being among the most distinctive.